Updated
Hollow Or Uneven Spots Under A New Floor
Part of the new floor sounds hollow, sinks slightly underfoot, or has a visible step between planks.
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What's happening?
A floor covering copies the subfloor beneath it. Where the base dips, a floating floor bridges the hollow and knocks when you step on it; where the base has a high spot, planks rock over it and the edges sit at different heights, which fitters call lippage. The Contract Flooring Association's guidance is that floor screeds and in-situ bases have to meet the surface regularity set out in the relevant British Standards, and that directly finished concrete frequently does not — mechanical preparation or a levelling compound is normally needed before a covering goes down. So a hollow spot is rarely a fault in the flooring product. It is the subfloor showing through, and it will keep showing through whatever is laid on top of it.
What usually causes it?
- A subfloor outside the flatness tolerance the covering requires, never levelled before laying
- A dip or high spot in a screed, or laitance left on a power-floated concrete base
- Old adhesive, paint or tile beds left in place under the new floor
- Underlay of the wrong thickness or type bridging hollows instead of following the base
- Chipboard deck joints stepped where the panels have swollen at the edges
- A previous covering removed and its ridges and residues not ground back
- Screed or levelling compound laid too thin and cracked or debonded under load
Is it dangerous?
No, but hollow spots are what open laminate joints. A joint that sits over a void hinges every time it is stepped on and eventually unclicks, so an unlevelled subfloor turns into visible plank separation within a year or two.
Can I fix it myself?
You can find them. Walk the floor slowly and mark every spot that knocks, then lay a long straightedge across the marked areas and look for daylight under it. That tells you whether the problem is a dip, a ridge or a stepped joint. What you cannot really do yourself is fix it without lifting the covering, because the correction has to happen in the subfloor, not on top of it.
When should you call a flooring fitter?
Call a flooring fitter when the hollow areas are spreading, when joints have already opened over them, when the floor was recently laid and is not performing, or before laying anything new over a base you suspect is uneven. A fitter will measure the subfloor against the tolerance the product needs, which is the difference between preparing once and re-laying twice.
What will the flooring fitter actually do?
- Lift the covering and survey the subfloor with a straightedge against the required surface regularity
- Grind or scarify high spots, laitance and old adhesive residues back to a sound surface
- Apply a primer and a levelling compound at the correct thickness to fill dips and correct falls
- Replace stepped or swollen deck panels rather than levelling over them
- Re-lay the covering with the right underlay and reinstate the perimeter expansion clearance
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